The present invention relates in general to a line thermal printer and, in particular, to a mechanism for supporting and raising the thermal head relative to the platen.
Thermal line printers are known in the art as disclosed in Japanese Utility Model Laid Open Application No. 164/85. This prior art printer includes a platen and thermal head. Heat sensitive paper is disposed between the platen and the thermal head. Rotation of the platen produces a frictional force for feeding the paper. When the heat sensitive paper is to be inserted into position, a lever is operated to actuate a head lifting mechanism to raise the thermal head to provide the required spacing between the platen and the thermal head to allow insertion of the paper. When the printer is not to be used for a long time, the head lifting mechanism is utilized to raise the thermal head from the platen to prevent deformation of the platen. For these purposes, the thermal head is raised a distance ranging from several millimeters to less than 10 millimeters.
To perform preventive maintenance to the thermal head, such as cleaning the thermal head surface, it is required that screws be removed and that the head support structure be disassembled or deformed. This exposes the surface of the thermal head to be maintained. In another method for servicing the head known from Japanese Laid Open Patent Application No. 134274/88, the entire thermal head is detached.
Generally, thermal line printers maintain the heat sensitive paper disposed along each print line and require an accuracy equal to the dot pitch. Therefore, print quality is greatly affected by the accuracy at which the heat generating portion of the thermal head is positioned relative to the platen. It follows that positioning accuracy is an important factor affecting print quality.
These prior art mechanisms have been satisfactory. However, it is difficult to accurately establish the position of the heat generating portion of the thermal head during printing once the thermal head has been repeatedly operated as is required in the above prior art structures. Continual disassembly or removal of the thermal head causes the positioning of the thermal head to deviate from the optimum position resulting in low print density. Additionally, the prior art thermal line printers are not easily operated. When a maintenance operation is to be performed, the head is disassembled, i.e., the screws are detached. This is a cumbersome operation to perform. During such an operation, there is an additional possibility that one of the screws which has been removed will fall into the printer, causing an electrical short circuit on the circuit board resulting in breakdown of the apparatus. Additionally, screws are easily lost. Once the head is disassembled, the heat generating portion of the head tends to shift from its proper position. Further, if the head must be frequently mounted and removed, then the connectors coupled with the head are easily damaged.
Many of the above described prior art mechanisms utilize an assembly for biasing the head toward the platen including spring members and structures for holding the spring members each of which are mounted near the top of the thermal head. Such a structure increases the total height of the printer resulting in a waste of space which prevents miniaturization of the printer. Additionally, when a paper cutter was incorporated into the printer, it was fixedly mounted to the printer, providing an impediment to the raising of the thermal head and greatly impeding access to the thermal head.
In many of the prior art thermal line printers, it is sometimes impossible to maintain the head, i.e. to clean the head due to the small space provided Even if it is possible, maintenance of the thermal head itself involves further cumbersome operations. Mechanisms which do lend themselves to maintenance are complicated structures which tend to be large in size.
Accordingly, it is desirable to provide a thermal line printer which overcomes the shortcomings of the prior art devices described above.